1. Field of the Invention
The present invention relates to a positive photoresist transfer material and a method for processing a surface of a substrate using the transfer material. More specifically, the present invention relates to a transfer material which is suitable for providing a photoresist layer on a substrate such as a TFT array substrate via a transfer method in order to conduct photoetching, and relates to a method of photoetching using the transfer material.
2. Description of the Related Art
As substrates for liquid crystal displays (LCDs), plasma display panels (PDPs) and the like have tended to be made larger and more precise, etching photoresists used in the manufacture of thin-film transistors (TFTs) or electrode plates have been required to have not only higher sensitivity, uniformity and resist saving ability but also adhesiveness to various substrates and dry-etching resistance (heat resistance). In particular, as substrates have been enlarged in recent years, there has been a strict demand for uniformity with regard to various points such as thickness and shape. For example, uniformity in thickness of the coating film between a central portion and a peripheral portion of the substrate has been required, and dimension uniformity due to demand for resolution enhancement has been required.
In conventional etching resists, an alkali-soluble phenol novolak resin, a 1,2-quinonediazide compound and a solvent are used as main components thereof, and a liquid composition comprising an adhesion accelerator, a coating aid, a colorant and the like is also used (Japanese Patent Application Laid-Open (JP-A) Nos. 6-27657 and 2000-105466). TFT array substrates and electrode plates for PDPs are manufactured by applying an etching resist to a conductive base material or an insulating base material which is sputtered on a glass or transparent plastic substrate, and then by subjecting a thin film of each layer to a process including drying, pattern exposure, development, etching and resist separation (hereinafter, referred to as a photoetching process). There are two methods for performing the etching: a wet-etching method using various liquid etchants, and a dry-etching method using ions or radicals (active type radicals) generated by decomposing gas with plasma in a pressure reducing device so as to vaporize and remove the film on the substrate.
As an example using the etching resist, a typical method for producing a TFT array substrate for an LCD will be described referring to FIG. 3 which shows a basic cross-sectional structure of the TFT. (1) A gate electrode 12a and a Cs electrode 12b are provided on a glass substrate 11 by using molybdenum tantalum (MoTa) or the like. (2) Then, a gate oxide film is formed on the gate electrode 12a by a silicon oxide (SiOx) film 13 and a nitride (SiNx) film 14. (3) An amorphous silicon (a-Si) layer 15 serving as a semiconductor activating layer is formed on the gate oxide film. (4) Further, an a-Si layer 16 mixed with N+ impurities for reducing bond resistance is provided on the a-Si layer 15. (5) Thereafter, a drain electrode 17a and a source electrode 17b are formed using a metal such as aluminum. The drain electrode 17a is connected to a data signal line, and the source electrode 17b is connected to a pixel electrode (or sub-pixel electrode) 19. (6) Finally, a protective film for protecting the a-Si layer 16, the drain electrode 17a and the source electrode 17b is provided via a nitride (SiNx) film 18.
A process for manufacturing a TFT array will now be described with reference to FIG. 4. First, in step (A), a metal film 22 for forming a gate electrode is sputtered on the entire surface of an insulating glass substrate 21. Possible examples of the metal include tantalum (Ta), aluminum (Al), alloys such as molybdenum tantalum (MoTa) and molybdenum tungsten (MoW), and the like.
Then, in step (B), a metal pattern 22a is formed by conducting photoresist application, drying, mask exposure, development and etching (hereinafter, this series of processes is simply referred to as a photoetching process). Thereafter, in step (C), a gate oxide (SiOx) film 23 is formed thereon by using a CVD technique.
Subsequently, by using the CVD technique, in step (D), a semiconductor (a-Si) film 24 is deposited thereon, and in step (E), a layer 25 to which a slight amount of phosphorus (N+) has been added is further formed thereon. Then, in step (F), only portions thereof that are to be included in the TFT are patterned via the photoetching process to form a semiconductor layer (a-Si film) 24a and a layer 25a. After that, in step (G), an ITO film 26, which is a transparent conductive film for a pixel electrode, is sputtered, and in step (H), a pixel electrode 26a is formed through the photoetching process.
Next, in order to form a power supply portion for a storage capacitor Cs, in step (I), a portion 23a of the gate oxide film 23 on the Cs is patterned and removed via the photoetching process. Subsequently, in step (J), a metal 27 such as aluminum or titanium is sputtered on portions of the TFT that are to be a drain electrode 27a and a source electrode 27b, and in step (K), the portions are patterned through the photoetching process to form the drain electrode 27a and the source electrode 27b. 
Finally, in order to protect elements such as the TFT, for example, a protective nitride (SiNx) film or the like is grown by the CVD. After being grown, the film is patterned through the photoetching process to form the protective film, and the TFT array is thereby completed.
In order to form the fine patterns by photoetching, a photoresist composition is generally used, which has two main components: an alkali-soluble novolak resin having a phenolic hydroxyl group, and a photosensitive substance having a 1,2-quinonediazide group as a photosensitive group. The coating thickness of the photoresist film is generally 0.5 to several micrometers.
The photoresist composition is used to form images having pattern dimensions widely ranged from about 0.3 micrometers (sub-half micron range) up to several tens or hundreds of micrometers, which enables fine process for surfaces of various substrates.
The photoresist composition is a positive photoresist which can be developed with alkaline water. The positive photoresist is more widely used than a negative photoresist such as rubber-type photoresist which requires being developed with a solvent, because in the case of the former, for example, the resolution is superior, the acid resistance and the etching resistance are more satisfactory, the problem of waste solution disposal is less serious (since no solvent is used in development), and most significantly because variation in image dimensions resulting from swelling during development is extremely small, whereby the dimension control is relatively easier.
As techniques for TFTs and STNs have been improved, line widths in LCDs and the like have tended to be made thinner and finer. For example, although design dimensions of elements using conventional TN or STN liquid crystals were 200 to several hundreds of micrometers, minimum design dimensions thereof have been reduced to at most 100 micrometers due to newly developed techniques. Further, minimum design dimensions of TFT display elements requiring excellent response ability or imaging ability have been reduced to a level of several micrometers.
The photoresist material has been required to maintain a capacity for fine processing and at the same time correspond to large areas. It has been important for enlarged substrates of liquid crystal displays and substrates, which were originally intended to be large, such as those of PDPs, to realize a uniform film thickness in the display.
In the case of large displays, it has commonly been required to further reduce costs and to decrease the amount of photoresist solution used.
In order to improve the uniformity of film thickness in the display and to decrease the amount of photoresist solution used, methods for coating have been continuously under study. As a result, in place of a conventional spin coater, a slit coater has been newly developed. However, while the slit coater holds promise for handling substrates having a size of up to 550 mm×680 mm, difficulties are forseen if it is to be applied to even larger substrates.
In a field of printed wiring boards, so-called dry film photoresists are widely used, which can correspond to substrates having a width of about 600 mm. The dry film photoresist material is produced by coating a polyester film having a thickness of 20 to 25 μm, which is a temporary support, with a negative photopolymerizable resin which can be developed with alkaline water, so as to generally have a thickness of 10 to 80 μm, and then overlaying a polyolefine film having a thickness of 4 to 20 μm thereon as a protective film. However, when the photoresist material is used in the field of printed wiring boards, a required resolution thereof is at most 30 to 300 μm. In general, the photoresist material is developed with an alkalescent aqueous solution which is typically a sodium carbonate aqueous solution having a concentration of 1%, a conductive material, generally copper, on the substrate is etched with a cupric chloride aqueous solution, and resist separation is conducted by using a caustic soda or caustic potash aqueous solution having a concentration of 2 to 3%.
Photoresists used for TFTs for LCDs, however, are required to correspond to the following conditions: high-resolution of 2 to 10 μm, metaion free development, separation via an organic separating solution, and undergoing an etching process for a metal thin film such as ITO, Ta or Al or for an inorganic thin film such as SiNx or ITO. In order to meet to these conditions, the photoresists are required to have a thickness of several micrometers, adhesiveness to various sputtered metal or inorganic thin film materials, uniform film thickness, an ability to follow preceding TFT patterns having deviations in thickness of about 1 μm, a capacity for high-speed lamination onto substrates having a width of 1 to 2 m, and the like. These conditions, however, completely exceed the limitations of conventional dry film resists. As described above, the conventional method of coating with the positive liquid resists or that of transferring the negative dry film photoresists cannot meet the needs of LCD and PDP fields.
A method for providing a color or colorless negative photoresist layer on a color filter substrate has been suggested, in which a thermoplastic resin layer, an intermediate layer and a negative photoresist layer are sequentially applied and dried on a film support to obtain a negative photoresist transfer material, and the negative photoresist transfer material obtained is transferred onto the color filter substrate, which has preceding pixels and deviations in thickness of about 2 μm (Japanese Patent Application Registration Nos. 2,794,242 and 2,873,889; and JP-A Nos. 10-97061 and 10-206888). In short, this is a method of forming a color filter or an overcoat layer, characterized in that a film having the negative photoresist layer is attached by using a lamination technique, and then patterning is conducted. In this method, while the negative photoresist layer, which is a thin film having a thickness of 1 to 5 μm, can be rapidly transferred onto the substrate having unevenness, the resolution is limited due to its being of the negative type. Therefore, although a photoresist layer of a positive type is desired, there have as yet been no practical positive photoresist layers successfully provided.
This is because most conventional positive photoresists comprise a phenol novolak resin as a main component and 1,2-quinonediazide sulfonate as a photosensitive component, and the films are therefore brittle and less flexible, which makes them difficult to produce as rolled products. Since rolled products of a desired width are generally obtained by slitting a wide roll on which the photoresist layer has been applied, if a brittle film has been applied thereon, unfavorable chips tend to be generated during the slitting. Further, the photoresist layer is generally heat-transferred onto a substrate to be processed by photolithography while the rolled film is being pressed thereon by a laminator. Therefore, chips may be also generated when the film is processed or cut in a longitudinal direction of the substrate after lamination. Since dust from the chips pollutes an operating environment of the substrate and the laminator, transfer layers with few defects have been difficult to obtain.
Conventionally, in the well-known photoresist material comprising the temporary support, the thermoplastic resin layer, the intermediate layer and the photosensitive layer, bubbles are generated when the temporary support and the thermoplastic resin layer are separated from each other or when the thermoplastic resin layer and the intermediate layer are separated from each other (for example, Japanese Patent Application Registration Nos. 2,794,242 and 2,873,889), because nitrogen gas (which results from photodecomposition of 1,2-quinonediazide) is generated after exposure. The bubbles unfavorably reduce the sensitivity and the sharpness of pattern edges.